The present invention relates to x-ray image intensifiers allowing x-ray imaging using low intensity x-rays. In particular, the invention relates to an image intensifier having a shortened form factor.
X-ray images are produced by projecting x-rays from an x-ray source, through an object to be imaged, to an x-ray detector. In x-ray fluoroscopy, x-rays are projected semi-continuously to a phosphor panel that emits light when struck by the x-rays.
In order to reduce the x-ray dose needed to produce such an image, modern fluoroscopy machines may use an image intensifier. A conventional image intensifier tube provides a front phosphor surface receiving x-rays and converting them to light. The light exits from the rear side of the phosphor surface and strikes a photo cathode, which converts the light to electrons that are discharged into an evacuated chamber behind the photo cathode.
High voltage plates, increasing the energy of individual electrons and/or their number, amplify the electrons. Focusing electrodes, micro channel plates, or other means may be used to preserve the spatial relationship of the electrons until the time they hit a target phosphor material to produce an optical image. This latter image is substantially brighter than that formed by the front phosphor surface. An electronic camera such as those using a charge-coupled device (CCD) may record the optical image to convert the optical image to electrical signals.
A drawback to image intensifiers is that they are relatively bulky structures often having a length several times the width of the entrance aperture defined by the front phosphor surface. X-ray machines using image intensifiers guide physicians in invasive procedures, such as catheterizations. A bulky image intensifier may interfere with the required repositioning of the x-ray machine during such procedures.
It is difficult to make the image intensifier shorter for a given entrance aperture. If the path over which the electrons are focused is reduced, the spatial distortion of the electrons increases requiring more complex focusing fields and structures. At some point the distortion compromises accurate guidance of medical instruments.
Micro channel plates can be used to amplify the electron stream without focusing it to a smaller area, eliminating the distortion of the electron focusing process. The large area image produced by such a micro channel system, however, must then be optically reduced by the lens system of the electronic camera. Short focal length optical reduction introduces distortion similar to those produced by short length electron focusing.